Adds 3 additional tests to test_flavor.py script

Adds three additional scenarios to flavor-manage functionality.
The test cases are added to the script test_flavors.py present in
"/tempest/tempest/tests/compute/admin" folder. Also adds try and
finally blocks to the already existing test cases.
Modified the "create_flavor" method in flavors_client.py and
made extra data as optional paramters instead of mandatory ones.

Change-Id: I8791dfc0cf1df6c1788b5c7e05f99e6eda989823
Implements: blueprint add-tests-flavor-manage
Use '-m' for tempest_coverage.py in tox.ini

Currently you can't run tempest_coverage.py because of an import
failure when trying to import from files in tempest. This is because
the python path from tempest/tools doesn't include it's parent
directory. This commit gets around the import failure with tox by
using 'python -m' for the script.

Change-Id: Iebbbbc1fbcf7f5fb0eb5d9dbdf3901cb77b90a43
Merge "Adds 3 additional tests to test_flavor.py script"
Merge "Use '-m' for tempest_coverage.py in tox.ini"